In order to achieve effective cleaning and removal of adhered substances or contaminants, including biofilm, proteins, carbohydrates, lipids, milk residues, deposits of food, beverages, contaminants of pharmaceuticals, including bio-pharmaceuticals and the like from equipment, piping and membrane surfaces, the adhesion forces between such contaminants and the surface to be cleaned must be overcome by the action of the cleaning process. To achieve good cleaning of such adhered residue or contaminants, the shear stresses generated by the cleaning process must be higher than the adhesive strength of the adhered contaminants to the surface to be cleaned. The simplest form of adhesion is due to van der Waals forces of attraction between the contaminant and the surface.
However, during actual industrial processing, other surface forces, such as electrostatic forces of attraction, acid-base interactions, hydrophobic forces, entanglement of contaminant molecules with roughness features of the substrate, or combinations of the above, are usually present between the surface of equipment or pipes and the contaminants to be removed. In these cases, the adhesion forces can become too high to be overcome with a simple circulation or flushing of cleaning liquids in the passageways, and thus cleaning cannot be achieved with such conventional means. When the contaminant is insoluble in the liquid employed in the cleaning operation, detachment of the contaminant from the surface and its subsequent flushing out from the pipeline, tubing and/or passageway are necessary to achieve good cleaning.
The physical nature of contaminants at a surface determines the extent and level of cleaning difficulty. The contaminant may be present on the surface as discrete particles or as layers of particles, in separate domains or areas covered by the contaminant. In the most difficult case, a continuous layer, as in the case of biofilm, food and dairy residues is present. Many cases of interest to the present invention relate to contaminants that are not soluble in the liquid or solution used in the cleaning process. The present invention is directed to cases when contaminants are mostly insoluble in the liquid used for a cleaning operation, when overcoming adhesion plays a considerable role in the cleaning process.
The conventional way to clean a pipeline, tubing or a passageway is to pass or circulate a liquid through the passageway. When the contaminant is present as discrete particles, or separate domains adhering to the surface, particle detachment, or contaminant domain detachment, by fluid (gas or liquid) flow must be achieved in order to clean the surface of the passageway. To achieve contaminant detachment, mechanical forces or shear stresses must be able to reach the contaminated surface. The ability to bring sufficient shear stress to the contaminated surface is a difficult task because of the fundamental limitations arising from the presence of a liquid boundary layer at the surface. The effect of the boundary layer on the ability to detach contaminants and clean surfaces of pipelines, tubing and passageways will be further explained below.
If the contaminant is present as discrete particles, and when there are several layers in the contaminant domain, it is possible to remove individual particles from the topmost layer of the contaminant domain. The removed particles then can be entrained and removed from the pipeline or passageway. It is possible that a whole section of the layer can be removed and entrained in a flowing fluid by a process called “denudation.” However, the contaminant layer may be left behind at the surface if the forces generated by the flow condition are not sufficient to detach the entire contaminant, especially with the limitation imposed by the presence of a liquid boundary layer at the surface. This is the case with conventional liquid circulation cleaning methods. Further, if the flow conditions are not sufficient to carry the detached contaminant out of the pipeline or passageway, the detached contaminants can deposit back onto the surface, and re-attach to the surface, or become entrapped in the boundary layer of the liquid near the surface. Therefore, it is necessary in order to achieve cleaning to provide flow conditions to transport the detached contaminants outside of the pipeline, tubing or passageway.
The conventional way to decrease the adhesive strength of a contaminant adhering to a surface is to use surfactants in the cleaning solution. Surfactant molecule may transport to the gap between the particle and the surface, and adsorb in the gap. The adsorption of surfactants increases the separation distance between the particle and the surface to be cleaned, and thus achieves a decrease in the adhesion strength of the particle to the surface, and thereby enhances detachment and transfer of the solid into the flowing fluid. The degree of detachment from the surface depends on the contact area between the contaminant and the surface to be cleaned. In the case of discrete particles attached to the surface, the contact area is small and detachment is possible. As the contact area between contaminant and surface increases, the total adhesion force become too large for liquid flow to achieve contaminant detachment, even in the presence of surfactants and conventional liquid flow rates. The most difficult contaminant to remove is when the contaminant covers most or even the entire surface to be cleaned, as in the case of biofilm, or a completely coated surface of food residues or other contaminants that are numerous in industrial processing, including pharmaceutical and biopharmaceutical residues.
When the contaminant covers the entire surface of a passageway, such as in the case of biofilm, milk or protein residues, and when the thickness of the contaminant layer is large, it is difficult for the surfactant to reach the interface between the contaminants and the surface, and therefore the adhesive strength remains high for cleaning with conventional liquid circulation, even if the cleaning solution includes surfactants and other cleaning ingredients. Furthermore, in the case of liquid circulation at 5 feet/sec, as in the conventional clean-in-place (hereinafter C-I-P) cleaning method, the shear stresses created at the surface are too small to detach biofilm or protein layers. This is due to the presence of thick boundary layers and other complex limitations due to fluid dynamics, and due to the difficulty of transfer of shear forces to the surface to be cleaned. This normally leads to lengthy cleaning times and to the use of high pH fluids, such as caustic and other harsh chemicals.
The final result is always insufficient for good and efficient cleaning. The use of liquid flow also demands large amounts of cleaning liquids, rinse water and other liquids used in the process of CIP cleaning. The result of such limitation is both economic and environmental, including loss of production time, the cost of expensive chemicals, and consumption of large amounts of water for rinsing operations, in addition to the cost of neutralization and discharge of the waste generated from such cleaning operations. Cleaning processes may in some cases produce more waste to discharge than the production operation itself, a scenario common in food, pharmaceuticals, biopharmaceuticals and other industrial processes.
The contact area between biofilm and tubing, pipeline or passageway surfaces that carry water or other processing liquids, is very large, since it almost covers the entire lumen surface as compared to the small contact area of a discrete particle attached to the surface. Correspondingly, the adhesion force of biofilm, or other similar contaminants that cover most or the entire lumen surface of a passageway, becomes very large. In order to achieve detachment and removal of biofilm or similar substances, the contaminant needs to be fragmented to create cracks or holes in the continuous contaminant layer so that surfactant diffusion to the interface between the contaminant and the surface of a passageway becomes possible.
Fragmentation of biofilm and like contaminants is believed to be necessary to allow surfactant diffusion and adsorption at the interface between the biofilm and the surface. The latter process is important for decreasing the adhesive strength of the biofilm to the lumen surface (interior surface) of a passageway. Otherwise, the adhesive strength of biofilm to solid surfaces such as glass, metal or plastic, as measured by many investigators, ranges from 50 to 120 Pascals, which is too high for conventional liquid circulation to overcome, even in the presence of surfactants. Therefore, fragmentation and crack formation of biofilm and like contaminant layers is needed to allow the decrease of the adhesion forces between biofilm and a surface to a level that is amenable to cleaning and provides sufficient shear stresses created by the flow conditions used in cleaning operations. This fragmentation and crack formation is almost impossible to achieve with conventional liquid circulation which is too slow for many applications.